1. Field of the Invention
The present invention relates to DC-DC converters including a soft-start circuit for use in switching power supplies, and to switching power supplies.
2. Description of Related Art
First Conventional Technology:
DC-DC converters including a soft-start circuit are used in electronic appliances, OA (office automation) appliances, etc. When an electronic appliance or the like starts up, the provision of a soft-start circuit prevents an overshoot in the output voltage of a DC-DC converter, and prevents malfunction of the electronic appliance or the like. It also prevents a rush current that may occur when the electronic appliance or the like starts up.
Japanese Patent Application published as No. 2012-114987 (hereinafter referred to as Patent Document 1) discloses a current-mode synchronous-rectification DC-DC converter including a soft-start circuit. During the soft-start period of the DC-DC converter, oscillation is started at a frequency lower than in the steady state, and the oscillation frequency is then increased gradually; this helps suppress fluctuation of the output voltage that tends to occur while the reference voltage is low, and makes it possible to bring the output voltage into a stable state more quickly.
Japanese Patent Application published as No. 2008-245431 (hereinafter referred to as Patent Document 2) discloses a DC-DC converter that achieves soft-start operation at start-up through digital control. The soft-start circuit disclosed in Patent Document 2 includes a frequency divider, an accumulator, and a DA (digital-to-analog) converter. The frequency divider divides the frequency of a frequency signal to output a frequency-divided signal. The accumulator operates based on a control signal fed from a CPU (central processing unit) and the frequency-divided signal. The DA converter generates a soft-start signal based on the output of the accumulator. This makes it possible to perform soft-start operation at start-up with no change in circuit configuration.
Japanese Patent Application published as No. 2006-288054 (hereinafter referred to as Patent Document 3) discloses a soft-start circuit that alleviates an overshoot in the output voltage by increasing the reference voltage stepwise with a counter at the start-up of a switching DC-DC converter.
Second Conventional Technology:
Conventionally, many switching power supply devices are provided with an overcurrent protection circuit that, on detecting an overcurrent, forcibly stops the switching operation of an output transistor. As overcurrent protection circuits, those of a pulse-by-pulse type are common, which repeat forcible suspension and self-recovery of switching operation every period. To cope with short-circuiting of the output to ground (short-circuiting to GND), in which case the overcurrent cannot be suppressed in time even with the ON-period of the output transistor reduced to the minimum value, there has also been proposed a technique of prolonging the forcible suspension period of switching operation by lowering the switching frequency with a view to enhancing the overcurrent suppression function of the pulse-by-pulse type configuration.
One example related to the conventional technology mentioned above is seen in WO 2010/134516.
Third Conventional Technology:
Conventionally, switching power supply devices that perform PWM (pulse width modulation) control of an output transistor based on the result of comparison between an error voltage and a slope voltage are used in various applications. The lighter the load (i.e., the lower the load current), the lower the error voltage, and thus the lower the ON-duty of the output transistor. Accordingly, some conventional switching power supply devices are provided with a function of stopping the switching operation of the output transistor during a period in which the error voltage is lower than the offset level of the slope voltage.
One example related to the conventional technology mentioned above is seen in Japanese Patent Application published as No. 2013-021816.
First Problem:
According to Patent Document 1, a reference voltage generator is provided which makes a reference voltage increase gradually at start-up and which keeps the reference voltage constant in steady operation. A reference voltage generator like this requires a complicated circuit configuration.
According to Patent Document 2, an overshoot is alleviated by dividing the frequency of a frequency signal with a frequency divider. However, when the frequency is switched, an overshoot occurs in the output voltage. Moreover, the use of the frequency divider results in a complicated circuit configuration.
According to Patent Document 3, an overshoot is alleviated by making a reference voltage increase stepwise with a counter. However, making the reference voltage increase stepwise produces not small overshoots. Moreover, the use of the counter results in a complicated circuit configuration.
Thus, an object of the present invention is to provide a DC-DC converter including a soft-start circuit that can satisfactorily suppress an overshoot in the output voltage at start-up without unduly complicating the circuit configuration.
Second Problem:
As for the second conventional technology, if the above-mentioned function of switching the switching frequency is introduced into a step-up switching power supply device of which the output voltage rises from the GND level, the switching of the switching frequency occurs right while the output voltage is rising. This inconveniently results in increased output ripples at start-up.
In view of the above-discussed problems encountered by the present inventors, another object of the present invention disclosed herein is to provide a switching power supply device that can enhance the overcurrent suppressing function of the pulse-by-pulse type configuration without increasing output ripples at start-up.
Third Problem:
As for the third conventional technology, in conventional switching power supply devices, the offset level of a slope voltage is fixed. This inconveniently results in the magnitude of the load current at the time of switching to a light-load mode varying depending on input/output settings (the difference between the input voltage and the output voltage).
In view of the above-discussed problems encountered by the present inventors, yet another object of the present invention disclosed herein is to provide a switching power supply device that can reduce variation of the load current at the time of switching to a light-load mode.